The invention relates to a device and a method of measuring a non-repeatable axial and/or radial runout of rotating components, particularly of the rotor of a spindle motor used, for example, to power hard disk drives.
Such spindle motors deployed in hard disk drives are used to power a magnetic disk on which data in the form of magnetized areas can be written and read out again by means of a read/write head.
At least one magnetic disk is secured to the rotor of the spindle motor. The rotor, driven by an electromagnetic rotating field, has a rotary bearing arranged in a bearing system. Roller bearings are preferably used in the bearing system, although fluid dynamic bearings may be used as well. Due to unavoidable component and assembly tolerances, there are substantial deviations between the actual and the theoretical rotational axis. Moreover, the position of the rotational axis varies over time, such that the distance between the rotor or its surface and a stationary point of reference does not show a constant measured value but rather a value changing over time. This value changes depending on the angle of rotation. This variation of the position of the rotational axis can lead to errors in reading and writing data onto the magnetic disk and, in the worst case, to a collision of the read/write head with the magnetic disk.
The runout of the rotor due to deviations in the actual rotational axis is defined in practice by the value of the RRO (Repeatable Runout), wherein a distinction should be made between RROaxial (axial runout) and RROradial (radial runout). RRO is a measurement of the deviation in the actual rotational axis due to an off-center alignment, tilt, surface imperfections or deviations in form due to the manufacturing process. For modern spindle motors used in hard disk drives, RRO is typically in the order of approximately 10 μm.
Superimposed on the RRO is the NRRO (Non Repeatable Runout), which results from random or stochastic deviations which, in contrast to the RRO, only appear irregularly with respect to both the phase relationship and its amplitude. NRRO is caused by irregularities stemming from the bearing system. In the case of roller bearings, for example, NRRO may result from irregularities in shapes of races and/or rollers, surface defects and diameter tolerances. For modern spindle motors used in hard disk drives, NRRO is typically in the order of approximately 10 nm and is thus several magnitudes smaller than RRO.
RRO and NRRO are measured with the aid of an appropriate measuring device. For this purpose, the spindle motor is set into a test station. Using a capacitive probe, where the distance of the probe from the rotor depends on the angle of rotation, both variables are measured while the motor is rotating. RRO and NRRO can be directly determined from the probe's output signal. The RRO typically generates a periodic signal represented as a sinusoid-shaped graph. The NRRO is superimposed on the sinusoid signal. FIG. 3 shows a variation of a typical test signal depending on the angle of rotation. The test signal essentially defines a sinusoid curve whose period corresponds to a rotational period of the motor under test.
Since the NRRO is much smaller than the RRO, the NRRO test signal is also very small in comparison to the RRO signal. The resolution of the measuring device is determined and set on the basis of the maximum amplitude of the RRO signal, so that the remaining resolution for the NRRO signal is extremely small. If the test signal is digitalized, for example with a resolution of 12 bit, the measured values of the NRRO are in the region of 1-2 bit. An exact measurement of the NRRO is thus only possible within limits. However, even then the resolution is inadequate.